When semiconductors are irradiated with light whose wavelength has an energy greater than a band gap thereof, an oxidation-reduction reaction is brought about. Such a semiconductor is called a photocatalytic semiconductor or merely a photocatalyst.
Photocatalysts are in the form of powder and may be used as suspended in a solution, or may be used as supported on a substrate. From the standpoint of photocatalytic activity, the former is more active owing to the greater surface area. From the standpoint of practical applications, it has been frequently experienced to inevitably adopt the latter rather than the former owing to the ease in handing.
In order to support a photocatalyst on a substrate, there has been adopted a method wherein the particles of a photocatalyst are sintered at high temperatures and supported on the substrate. Another method has been proposed wherein a certain type of fluoropolymer is used as a binder, with which a photocatalyst is supported on a substrate. For instance, Japanese Laid-open Patent Application No. 4-284851 sets out a method wherein a mixture of the particles of a photocatalyst and a fluoropolymer is built up as layers and bonded under compression pressure. Japanese Laid-open Patent Application No. 4-334552 sets forth a method wherein the particles of a photocatalyst are thermally bonded to a fluoropolymer. Moreover, Japanese Laid-open Patent Application No. 7-171408 sets out a method wherein the particles of a photocatalyst is bonded on a substrate through a hard-to-decompose binder including an inorganic binder such as water glass or an organic binder such as a silicone copolymer, and also a method for manufacturing a photocatalytic body which includes, on a substrate, a first layer made of a hard-to-decompose binder, and a second layer formed on the first layer and made of a hard-to-decompose binder and the particles of a photocatalyst. In addition, Japanese Laid-open Patent Application No. 5-309267 describes a method wherein the metal oxide obtained from a metal oxide sol is used to support and fix the powder of a photocatalyst therewith. It is stated that the metal oxide sols are obtained from organometallic compounds such as alkoxides, acetylacetonate, carboxylates of metals as used in a sol-gel method, or are obtained by hydrolysis of an alcohol solution of chlorides, such as titanium tetrachloride, in the presence of an acid or alkali catalyst.